In an unexpected discovery, hundreds of gas plumes bubbling up from the seafloor were spotted during a sweeping survey of the U.S. Atlantic Coast.

Even though ocean explorers have yet to test the gas, the bubbles are almost certainly methane, researchers report today (Aug. 24) in the journal Nature Geoscience.

"We don't know of any explanation that fits as well as methane," said lead study author Adam Skarke, a geologist at Mississippi State University in Mississippi State.

Surprising seeps

Between North Carolina's Cape Hatteras and Massachusetts' Georges Bank, 570 methane seeps cluster in about eight regions, according to sonar and video gathered by the National Oceanographic and Atmospheric Administration ship Okeanos Explorer between 2011 and 2013. The vast majority of the seeps dot the continental slope break, where the seafloor topography swoops down toward the Atlantic Ocean basin. [Gallery: Amazing images of Atlantic Methane Seeps]

The Okeanos Explorer used sound waves to detect the methane bubbles and map the seafloor. The technique, called multibeam sonar, calculates the time and distance it takes for sound waves to travel from the ship to the seafloor and back. The sonar can also detect the density contrast between gas bubbles and seawater.

Huge canyons etched in the shallow continental shelf also hide bubble plumes, as well as diverse ecosystems that are based on methane-loving bacteria. In 2013, researchers explored a handful of these seeps with Jason, a remotely operated vehicle, finding them teeming with crabs, fish and mussel beds. In Norfolk canyon off the coast of Virginia, researchers from the University of North Carolina at Wilmington discovered the largest methane seep ever found in the Atlantic Ocean, and possibly all the world's oceans. [Photos: Unique Life Found at East Coast Gas Seep]

Most of the methane seeps are in water less than 1,640 feet (500 meters) deep. Most of these shallow methane seeps seem to arise from microbes blurping out methane, the researchers said. The researchers did find some deeper methane vents, at which the ROV Jason glimpsed patches of methane hydrate. This is the icy mix of methane and water that appears when deep ocean pressures and cold temperatures force methane to solidify. Any type of methane gas can form hydrates.

An illustration of the Atlantic margin showing the relationship between methane seeps and seafloor features.

Credit: A. Skarke and C. Ruppel/Nature Geoscience

While methane vents are common around the world, only three natural gas seeps — where methane escapes from seafloor sediments — had been found off the East Coast before 2012.

"It was a surprise to find these features," Skarke said. "It was unexpected because many of the common things associated with methane gas do not exist on the Atlantic margin."

Gas, gas, gas?

The East Coast is a passive margin, and methane isn't expected to come out of this environment. The margin hasn't been squeezed or pulled by plate tectonic activity for tens of millions of years, and that means a lack of escape routes for methane. "I usually describe passive margins as cold, old and boring," said study co-author Carolyn Ruppel, chief of the U.S. Geological Survey gas hydrates project in Woods Hole, Massachusetts. [In Images: How North America Grew as a Continent]

Also missing from the Atlantic Coast are layers of salt, which are responsible for the Gulf of Mexico's oil and gas.

Without more exploring, the researchers can't say for sure why there are so many methane plumes along the Atlantic coastline. "It's a huge research area that needs to be pursued," Ruppel said.

If the East Coast could hide hundreds of bubbling methane pits, then it's likely there are nearly 30,000 more awaiting discovery in the world's oceans, the researchers said.

"These processes may be happening in places we didn't expect them," Skarke said.

Cup corals and bubblegum corals live on rock near the edge of the mussel bed.

There's also a good chance more methane vents will be found off the East Coast, but that doesn't mean one should expect new drilling platforms popping up offshore to extract the gas, the researchers said. "We have no evidence to suggest this material would be a recoverable resource," Skarke told Live Science. "There is no evidence whatsoever that there are conventional deep-seated oil and gas reservoirs underneath the Atlantic margin."

The more likely scenario: A fleet of research ships hurries to claim the seeps. The methane seeps are near ports where many of the U.S. research ships dock. The ease of access has set off an exploration stampede, with several new projects in planning stages or already funded.

"We're setting the stage for a decade of discovery," Ruppel said.

From the Arctic to Atlantic

Interest is running high because the seeps could be a laboratory for studying how methane hydrates respond to climate change.

Methane is a greenhouse gas that disappears more quickly than carbon dioxide in the atmosphere, but has more warming power than carbon dioxide. Millions of tons of methane are frozen in Arctic permafrost, both on land and in the seafloor. Recently, several studies have warned that rapid warming in the Arctic could upset these deposits, melting them and freeing the gas. This would boost the planet's greenhouse gas levels and could accelerate climate change.

A close-up of methane hydrate observed at a depth of 3,460 feet (1,055 meters) off the U.S. Atlantic Coast.

"Now we have a study site where we can monitor these locations and see how they change," said David Valentine, a geochemist at the University of California, Santa Barbara, who was not involved in the study. "Finally we have a place where we can begin to address some of the questions about how water temperatures are influencing methane."

At present, scientists think the East Coast seeps don't contribute much methane to climate change.

Most of the methane gas dissolves in the ocean before reaching the surface, Ruppel said. The total amount of gas is also much smaller than sources on land, such as cows or gas drilling. "It's probably on the order of a feedlot of methane," Valentine said. However, some shallow-water seeps could vent methane to the surface, and researchers expect that future surveys will uncover even more shallow seeps. These regions only received a cursory look during the survey.

Even though the methane may not escape to the atmosphere, the gas still adds to the ocean's overall carbon budget — which is still a wildly uncertain number.

"It's not a huge number, but it's an important number for us to know," Ruppel said.

Becky Oskin covers Earth science, climate change and space, as well as general science topics. Becky was a science reporter at Live Science and The Pasadena Star-News; she has freelanced for New Scientist and the American Institute of Physics. She earned a master's degree in geology from Caltech, a bachelor's degree from Washington State University, and a graduate certificate in science writing from the University of California, Santa Cruz.